1. Field of the Invention
The present invention relates to a non-oriented electromagnetic steel sheet having excellent magnetic properties after stress relief annealing and a method of manufacturing the same.
2. Description of the Related Art
Non-oriented electromagnetic steel sheets have been used as the iron core materials of motors, transformers, and the like. It is desirable to lower the iron loss of the non-oriented electromagnetic steel sheet in order to increase the energy efficiency of these devices.
Recently, it has become especially important to make the motors more efficient. Accordingly, it is desired to improve the magnetic properties of the non-oriented electromagnetic steel sheet, in particular, to increase its magnetic flux density and lower its iron loss. Also, the rotor unit thickness of a DC brushless motor, for example, is reduced up to about 5 mm by embedding a permanent magnet into a rotor. Accordingly, adequate mechanical strength, which has not been important in conventional small motors, is also required for the non-oriented electromagnetic steel sheet, in addition to the magnetic properties. That is, there is required an electromagnetic steel having excellent magnetic properties and adequate mechanical strength as a material for small motors of high efficiency.
As a means for reducing the iron loss of the non-oriented electromagnetic steel sheet, there is available a method of optimizing a grain size and a method of improving the specific resistance of the steel sheet. That is, it is well known that the iron loss is minimized by the grain size of about 150-200 .mu.m, that the addition of Si or Al is effective to improve a specific resistance, and that mechanical properties depend on Si and Al in steel.
On the other hand, it is also well known that a problem arises in that a saturation magnetic flux density is reduced and the punching property of steel sheet is deteriorated when the content of Si or Al is increased. In particular, the punching property is a very important property for the non-oriented electromagnetic steel sheet. Non-oriented electromagnetic steel sheet is often used by users after it is punched to a prescribed shape and then subjected to stress relief annealing. Since the punched shape is complicated and requires accuracy, a precise punching property is required for the non-oriented electromagnetic steel sheet. The punching property is deteriorated by the increase of the hardness and grain size of the electromagnetic steel sheet. The increase of the hardness and grain size results from an increase in the alloying components of the steel sheet or scales formed on the surface of the steel sheet. For example, when Si exceeds 1.0 wt % or when the grain size of a finished steel sheet exceeds 40 .mu.m, a problem arises in that the punching property is deteriorated.
Accordingly, the recent demand for higher motor efficiency and adequate mechanical strength requires a material having an excellent grain-growing property after stress relief annealing, which thereby has a high magnetic flux density and very low final iron loss without sacrificing its punching property.
This need can be met by sufficiently increasing the Si and Al content thereby to coarsen the crystal grains. In particular, it is preferred to increase the content of Al because it has less effect on increased hardness. Also, it is preferred to coarsen crystal grains because it reduces the iron loss after stress relief annealing. More specifically, although Si and Al have the same degree of specific resistance increasing effect, the Al content is increased because the effect of Al per unit weight on the increase of hardness is about one half that of Si. On the other hand, although increasing the stress relief annealing temperature is effective to coarsen crystal grains, the grain growing property must be improved in a relatively low stress relief temperature region of about 750.degree. C. at the highest, which is employed in practice due to cost considerations.
Japanese Unexamined Patent Publication No. 8-3699 discloses a low Si non-oriented electromagnetic steel sheet in which a Si component is lowered to 1.0 wt % or less to obtain an excellent growing property after stress relief annealing and a low final iron loss. The grain growing property of the non-oriented electromagnetic steel sheet is greatly improved by adding REM (rare earth metals) to the steel and highly purifying the steel during the steel-making process. High purity is accomplished by suppressing the contents of Ti and Zr, which are elements contained in a trace amount. Precipitates which deteriorate grain growth are controlled by REM-addition or purification. According to the publication, this works remarkably well; however, due to the low Si content, the problem arises that the mechanical strength is insufficient for some locations where the steel sheet is used and iron loss of the sheet is insufficient to meet the need for a greater reduction in iron loss of the cores.
Japanese Examined Patent Publication No. 61-4892 seeks to improve magnetic properties by increasing the Al content. However, although mechanical properties were improved by the increase of only the Al content, the magnetic properties were greatly altered. In particular, a low loss product could not be obtained stably after stress relief annealing as described later. It has been found by the inventors that the above problem is caused by nitriding during the stress relief annealing.
In Japanese Unexamined Patent Publication No. 8-296007, it is disclosed that the deterioration of the magnetic properties of a steel sheet containing a large content of Al is suppressed by controlling C contained in an insulating film, because the deterioration is caused by nitriding in stress relief annealing. According to the publication, however, although the change in magnetic properties is reduced, the degree of reduction remains insufficient and it is necessary to suppress the change altogether.